An example of a conventional flyback converter type switching power source is shown in FIG. 5. In the switching power source, when current flowing through a secondary coil Ns of a transformer T1 becomes zero, a voltage reversal at a control coil Nc of the transformer T1 is detected, whereby frequency variable control which turns on a main switch Q1 is performed.
Furthermore, when the current of the secondary coil Ns of the transformer T1 becomes zero, the voltage reversal of the control coil Nc of the transformer T1 is detected, and using resonance caused by a primary coil Np of the transformer T1 and a capacitor which are connected in series with or in parallel to each other, control is performed in order to realize reduction of switching loss and low noise thereof by performing delay adjustment of on-timing of the main switch Q1, that is, the so-called soft switching operation is performed (refer to Laid Open Patent No. H09-205771).
Moreover, in such a flyback converter system for a switching power source, an oscillation frequency thereof is fixed, and a PWM control, which is based on an on-off ratio, is performed, in which a miniaturization of a transformer is realized by operating it with a switching current waveform which is a trapezoid current waveform.